1. Field of the Invention
The present invention relates to a cam grinding device and a cam grinding method. Specifically, the present invention relates to a grinding device for a composite cam in which two cams having different cam lift amounts and different phase angles are arranged adjacently in the axial direction and a cam grinding method thereof.
2. Description of the Related Art
Suction into and discharge from cylinders of an internal combustion engine are performed by valve-opening operation of valves. This valve-opening operation of valves is performed by operation of cams that rotate.
In the valve-opening operation of valves, from a viewpoint of improving output of the internal combustion engine, for example, valve-opening operation control is performed differently between at a low-speed rotation and at a high-speed rotation of the internal combustion engine.
In one of such control methods, a first cam for low speed and a second cam for high speed are provided as cams that actuate each valve, and valve-opening control is performed by appropriately selecting the first cam and the second cam in accordance with the rotational speed of the internal combustion engine. In this case, selective switching between the first cam and the second cam is performed by relative movement of a tappet of the valve between the first cam and the second cam in the axial direction while the tappet is in contact with the cams.
FIG. 13 to FIG. 15 are schematic diagrams illustrating positional relations between a first cam 112 for low speed and a second cam 114 for high speed. As can be seen from these schematic diagrams, in general, the maximum lift height of the first cam 112 for low speed is set low, and the maximum lift height of the second cam 114 for high speed is set greater than that of the first cam 112. The phase angles of both cams 112 and 114 are set such that the second cam 114 for high speed precedes in phase the first cam 112 for low speed in the rotation direction (arrow direction in FIG. 13), that is, valve-opening operation of a valve at the second cam 114 is performed earlier. Accordingly, as depicted in FIG. 13, the cam profile of the second cam 114 for high speed in the lift-height direction and the cam profile of the first cam 112 for low speed in the lift-height direction are offset from each other in the angular direction, and are in a positional relation in which these cams protrude from each other when viewed from the cam axial direction.
As depicted in FIG. 14 and FIG. 15, the first cam 112 for low speed and the second cam 114 for high speed are arranged adjacently in the axial direction. In other words, both cams are arranged as a composite cam 110. In this case, base circular portions of the first cam 112 for low speed and the second cam 114 for high speed are both formed by a constant radius r from a camshaft center. A certain angular range in which the base circular portions of both cams overlap is a common-base circular portion C. In the range of this common-base circular portion C, the relative movement of the tappet between the first cam 112 and the second cam 114 while the tappet is in contact with the cams is performed.
Cam grinding of the composite cam 110 including the first cam 112 for low speed and the second cam 114 for high speed is generally performed with a grinding wheel T (see FIG. 14 and FIG. 15) in a cam grinding device. In this grinding of the composite cam 110, after one cam of the first cam 112 and the second cam 114 is plunge ground, the other cam is plunge ground.
For example, the case depicted in FIG. 14 and FIG. 15 is a case in which the first cam 112 for low speed is ground first and the second cam 114 for high speed is ground later. In this case, the first cam 112 is ground with the grinding wheel T on the basis of predetermined cam lift data of the first cam 112 for low speed. Subsequently, the grinding wheel T is moved to a position corresponding to the second cam 114 for high speed, and the second cam 114 is ground with the grinding wheel T on the basis of predetermined cam lift data of the second cam 114 for high speed. In this manner, the composite cam 110 is ground by the cam grinding device. For example, see the specification of German Patent No. 10333916 and Japanese Patent Application Publication No. H4-13560.
In the grinding of the composite cam 110 with the grinding wheel T performed by the cam grinding device described above, as depicted in FIG. 15, at a boundary portion between the first cam 112 and the second cam 114 in the range of the common-base circular portion of the first cam 112 and the second cam 114, an unground portion F that is left unground is problematically generated. Illustrations of the unground portion F depicted in FIG. 14 and FIG. 15 are illustrated in an exaggerated manner for the purpose of easy understanding. Specifically, the unground portion F has a size on the order of several micrometers.
When the unground portion F exists at the boundary portion between the first cam 112 and the second cam 114 in the range of the common-base circular portion C, relative movement of the tappet between the first cam 112 and the second cam 114 requires the tappet to get over this unground portion F. This makes it difficult to perform this movement smoothly, thereby affecting the valve-opening control of the valve. Thus, the grinding wheel T needs to be trued frequently.
The following specifically describes the problem that the unground portion F is generated. In general, as depicted in FIG. 14 and FIG. 15, the axial direction width of the grinding wheel T is greater than the axial direction widths of the first cam 112 for low speed and the second cam 114 for high speed. Both ends Ta and Tb of the grinding wheel T on the grinding surface side become blunt by grinding the cams as workpieces. Specifically, both ends Ta and Tb wear sooner than a central portion, and the blunting is caused.
In view of this, herein, as depicted in FIG. 14, when the first cam 112 for low speed is plunge ground, the grinding wheel T is positioned so that the right end Ta thereof is aligned with the boundary portion between the first cam 112 and the second cam 114. Accordingly, the left end Tb of the grinding wheel T is positioned projecting beyond the left side of the first cam 112. Thus, the blunting at the left end Tb of the grinding wheel T does not affect the grinding of the first cam 112. However, the blunting at the right end Ta of the grinding wheel T affects the grinding on the first cam side at the boundary portion between the first cam 112 and the second cam 114, thereby causing an unground portion F to remain. The black-filled portion in FIG. 14 is the unground portion F. In FIG. 14 and FIG. 15, grinding allowances of the first cam 112 and the second cam 114 are indicated by imaginary lines. It should be noted that these lines are also drawn in an exaggerated manner for the purpose of easy understanding.
Subsequently, after the grinding of the first cam 112 is completed, as depicted in FIG. 15, the grinding wheel T is relatively moved to the position of the second cam 114, and the second cam 114 is plunge ground with the grinding wheel T. In this plunge grinding, the grinding wheel T is positioned so that the left end Tb thereof is aligned with the boundary portion between the first cam 112 and the second cam 114. Accordingly, the right end Ta of the grinding wheel T is positioned projecting beyond the right side of the second cam 114, and thus the blunting at the right side of the grinding wheel T does not affect the grinding of the second cam 114. However, the blunting at the left end Tb of the grinding wheel T affects the grinding on the second cam side at the boundary portion between the first cam 112 and the second cam 114, thereby causing an unground portion F to remain. This unground portion F in FIG. 15 and the unground portion F of the first cam 112 in FIG. 14 are both illustrated, filled with black, and remain at the boundary portion between the first cam 112 and the second cam 114.